Adaptive synchronous rectifier control

ABSTRACT

A switching power converter may include a power switch coupled to a primary winding of a transformer, and a primary controller configured to turn on and off the power switch, a synchronous rectifier switch coupled to a secondary winding of a transformer, and a synchronous rectifier controller configured to turn on and off the synchronous rectifier switch. The synchronous rectifier controller may monitor a voltage across the synchronous rectifier switch. The synchronous rectifier controller may detect a fault condition responsive to the voltage reaching a turn-off voltage threshold before a minimum on-time timer expires. The synchronous rectifier controller may detect a fault condition responsive to the synchronous rectifier switch being turned off at the same time, immediately after, or within a timing guardband after the minimum on-time timer expires. The synchronous rectifier controller may adaptively increase a minimum off-time period for the synchronous rectifier switch.

TECHNICAL FIELD

This application relates to switching power converters, and moreparticularly to switching power converters that use synchronousrectification,

BACKGROUND

The explosive growth in mobile electronic devices such as smartphonesand tablets creates an increasing need in the art for compact andefficient switching power converters so that users may recharge thesedevices. A flyback switching power converter is typically provided witha mobile device as its transformer provides safe isolation from AChousehold current. A conventional flyback converter that uses arectifying diode at the secondary (load) side of its transformer hassignificant power loss due to a relatively high forward voltage drop inthe rectifying diode. Thus, synchronous rectification techniques havebeen developed that replace the rectifying diode with an activelycontrolled switch such as a field-effect transistor (FET) device (e.g.,a metal oxide field-effect transistors (MOSFET) device) to improveoperating efficiencies by taking advantage of its lower power losses.

Conventional flyback converters with synchronous rectification typicallyinclude a synchronous rectifier controller that controls the synchronousrectifier switch (S2) based on a voltage across the synchronousrectifier switch terminals. When this voltage falls below an on-timethreshold voltage, the controller switches on the synchronous rectifierswitch S2 so that power is delivered to load. During this power deliver,the voltage across the synchronous rectifier switch S2 gradually risesabove the on-time threshold voltage until it crosses an off-timethreshold voltage. This off-time threshold voltage corresponds to thevoltage across the synchronous rectifier switch at the transformer resettime when the secondary winding current has ramped down to zero.

The timing of the on and off states for the synchronous rectifier switchis critical to reduce losses. But the control with regard to the on-timeand off-time threshold voltages is problematic because the voltageacross the synchronous rectifier switch S2 will have a resonantoscillation at the switch on times and off times due to parasiticeffects. When the synchronous rectifier switch S2 is switched on, thisresonant ringing could cause the switch voltage to exceed the off-timethreshold voltage such that the controller would undesirably switch offthe synchronous rectifier switch even though the secondary windingcurrent is still relatively robust (it being well before the transformerreset time). To prevent such an undesirable premature cycling off of thesynchronous rectifier switch S2, it is conventional for the controllerto apply a minimum on-time period with regard to monitoring the off-timethreshold voltage following the cycling on of the synchronous rectifierswitch S2. During this minimum on-time period, the controller does notrespond to the synchronous rectifier switch S2 voltage exceeding theoff-time threshold voltage.

An analogous minimum off-time period follows the cycling off of thesynchronous rectifier switch S2 to prevent the controller fromresponding to a resonant oscillation of the switch voltage that causesthe switch voltage to fall below the on-time threshold voltage. But incontrast to the resonant oscillation that occurs at the synchronousrectifier switch S2 on-time, the resonant oscillation at the switchoff-time is markedly more robust and prolonged. This robust off-timeoscillation of the switch voltage complicates the setting of anappropriate duration for the minimum off-time period that may be betterappreciated through a consideration of the waveforms shown in FIG. 1 fora power cycle 110 and a power cycle 120 of a primary-side power switchS1. In this example, the voltage across the synchronous rectifier switchS2 is a drain-to-source (V_(D-S)) voltage for a MOSFET. In response tothe cycling off of power switch S1, the drain-to-source voltage forsynchronous rectifier switch S2 falls below the on-time thresholdvoltage. The synchronous rectifier switch S2 is thus switched on whileat the same time a timer (S2 Min T_(ON) Timer) is started to time theminimum on-time period. The resulting resonant oscillation of thedrain-to-source voltage is relatively minor and quickly damped such thatthe duration of the minimum on-time period may be relatively short.

In response to the cycling on of the synchronous rectifier switch S2,the secondary winding current pulses on and begins to ramp down until itreaches zero at the transformer reset time (T1 Reset). At the same time,the drain-to-source voltage for switch S2 rises above the off-timethreshold voltage (S2 OFF Threshold) such that the S2 switch is switchedoff and a timer (S2 MIN T_(OFF) Timer) begins timing the minimumoff-time period, The resulting resonant oscillation for thedrain-to-source voltage following the synchronous rectifier switch S2off time is more pronounced and slower to damp as compared to thedamping that occurs at the on time for synchronous rectifier switch S2.For power cycle 110, the minimum off-time period has a proper durationsuch that the resonant oscillations of the drain-to-source voltage donot cross the on-time threshold voltage following the termination of theminimum off-time period.

But the resonant oscillations following the synchronous rectifier switchS2 off time are more pronounced for a subsequent power cycle 120 ofpower switch S1. Due to this more pronounced resonance, thedrain-to-source voltage crosses the on-time threshold voltage at a time122 following the termination of the minimum off-time period in powercycle 120. As a result, the controller cycles the synchronous rectifierswitch S2 on despite there being no power pulse to deliver. The resultis that the secondary winding current has a slightly negative valueduring the minimum on-time period following time 122. Upon thetermination of this undesirable minimum on-time period, thedrain-to-source voltage exceeds the off-time threshold voltage such thatthe synchronous rectifier switch S2 is cycled off for another minimumoff-time period. But the subsequent resonant oscillation of thedrain-to-source voltage again causes the drain-to-source voltage tocross the on-time threshold voltage such the synchronous rectifierswitch S2 is again cycled on a time 124. Another negative current isinduced on the secondary winding until the termination of the subsequentminimum on-time period whereupon the drain-to-source voltage againexceeds the off-time threshold voltage such the synchronous rectifierswitch S2 is opened.

The resulting cycling on and off of the synchronous rectifier switch S2following the transformer reset time is undesirable for a number ofreasons. For example, the negative current excited across the secondarywinding wastes power. More fundamentally, the synchronous rectifierswitch S2 may be cycled on when the power switch cycles on, which is asevere problem. The prior art setting of the minimum off-time period isthus problematic in that it cannot be set too short or this undesirablecycling of the synchronous rectifier switch S2 occurs yet it cannot beset too long in that the minimum off-time period would then interferewith the next power switch S1 cycling.

Accordingly, there is a need in the art for improved synchronousrectifier control techniques for switching power converters,

SUMMARY

To address the need in the art for improved synchronous rectificationtechniques, a switching power converter is provided with a synchronousrectifier controller configured to monitor a duration on an on-timeperiod for a synchronous rectifier switch. If the duration is too short,the synchronous rectifier controller increases a duration of a minimumoff-time period for the synchronous rectifier switch to address theresulting fault detection. In this fashion, resonant oscillation of avoltage across the synchronous rectifier switch is prevented fromcausing the synchronous rectifier controller to repeatedly cycle thesynchronous rectifier switch on and off in between on-times for a powerswitch.

In particular, it is desirable that the synchronous rectifier controllermaintain the synchronous rectifier switch off following a transformerreset time and prior to a subsequent cycling on of the power switch. Butdue to the resonant oscillations of the voltage across the synchronousrectifier switch after it is switched off, conventional synchronousrectifier controllers would undesirably cycle the resonant switch onprior to the cycling on of the power switch. The voltage across thesynchronous rectifier switch would then promptly cross the off-timethreshold voltage to trigger a cycling off of the synchronous rectifierswitch following the expiration of the minimum on-time period forsynchronous rectifier switch. This cycling off of the synchronousrectifier switch then triggers yet another resonant oscillation of thevoltage across the synchronous rectifier switch, which in turn raisesthe danger of yet another undesirable cycling on of the synchronousrectifier switch. In this fashion, the synchronous rectifier switchcould continue to cycle on and off such that the normal power deliveryto the load is disrupted.

To address this problem, it was known to adjust the minimum off-timeperiod by monitoring the voltage across the synchronous rectifier switchduring the duration of the minimum off-time period. But suchconventional techniques are undesirable in that minimum off-time periodmay be extended such that the cycling of the power switch is missed. Incontrast, the disclosed monitoring of the minimum on-time periodadvantageously prevents the undesirable repeated cycling on of thesynchronous rectifier switch subsequent to the transformer reset timeand prior to the cycling off of the power switch. These advantageousfeatures may be better appreciated through a consideration of thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates waveforms for a conventional flyback converter withsynchronous rectifier control.

FIG. 2 is a diagram of a flyback converter configured for adaptivesynchronous rectifier control in accordance with an embodiment of thedisclosure.

FIG. 3 illustrates waveforms for a flyback converter with adaptivesynchronous rectifier control in accordance with an embodiment of thedisclosure.

FIG. 4 is a flowchart for an example method of operation in accordancewith an embodiment of the disclosure.

Embodiments of the present disclosure and their advantages are bestunderstood by referring to the detailed description that follows, Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures.

DETAILED DESCRIPTION

Systems, devices, and methods are provided that allows for acycle-by-cycle adaptive setting of a minimum off-time timer forsynchronous rectification.

An example flyback converter 200 configured for adaptive synchronousrectifier control is shown in FIG. 2. In various embodiments, flybackconverter 200 includes a primary side such as a power stage 210 and asecondary side such as a secondary output stage 220. Power stage 210 mayinclude a power switch S1, a primary controller U1 configured to controlthe on state and the off state of power switch S1, and primary windingsNp of a transformer T1. Controller U1 may maintain the output regulationof flyback converter 200 by controlling the on and off states of powerswitch S1. Secondary output stage 220 may include a synchronousrectifier switch S2, an adaptive synchronous rectifier controller U2configured to control the on state and the off state of a synchronousrectifier switch S2, secondary windings Ns of transformer T1, and anoutput capacitor C1. Power switch S1 and synchronous rectifier switch S2may each be a field-effect transistor (FET) device (e.g., a metal oxidefield-effect transistors (MOSFET) device), a bipolar junction transistor(BJT) device, or other appropriate switch.

In some embodiments, adaptive synchronous rectifier controller U2includes a voltage sensor 222, a minimum on-time timer 224, and acontrol logic circuit 226. Voltage sensor 222 may be configured tomonitor a voltage across synchronous rectifier switch S2 by sensingthrough one or more terminals of adaptive synchronous rectifiercontroller U2. Timer 224 may comprise analog or digital circuitry.Control circuit 226 may comprise logic gates or a microcontroller.Control circuit 226 may be configured to turn on synchronous rectifierswitch S2 for at least a minimum on-time period responsive to thevoltage sensed by voltage sensor 222 crossing an on-time thresholdvoltage. Similarly, control circuit 226 may be configured to turn offsynchronous rectifier switch S2 for at least a minimum off-time periodresponsive to the sensed voltage crossing an off-time threshold voltage.By comparing a duration of each on-time period to the duration of theminimum on-timer period as timed by tiimer 224, control circuit 226 maydetermine when the duration of a given on-time period is too short so asto trigger a detection of a fault condition. In one embodiment, such a“too short” duration may equal the duration of the minimum on-timeperiod. In alternative embodiments, the duration of an on-timer periodmust be less than or equal to a sum of the minimum on-time period and aguard band period.

In response to the detection of the fault condition, control circuit 226may adaptively increase the minimum off-time period. This increase ofthe minimum off-time period may be limited to not exceed a maximumallowable off-time period such that the synchronous rectifier controllerU2 may be ready to switch on the synchronous rectifier switch S2 inresponse to a power switch S1being cycled off. In further embodiments,voltage sensor 222, timer 224, and a controller circuit 226 may beimplemented using a combination of hardware, software, and/or firmwarecomponents.

In various embodiments, when power switch S1 is placed in the on state,an input voltage V_IN drives the a primary current into the primarywindings Np of transformer T. Based upon the input voltage V_IN and amagnetizing inductance for transformer T1, the primary current ramps upfrom zero Amperes (Amps) to a peak current value, whereupon controllerU1 turns off power switch S1 to complete a power cycle.

In various embodiments, synchronous rectifier switch S2 is placed in theon state when power switch S1 is placed in the off state in order todeliver energy stored in transformer T1 to secondary output stage 220,and placed in the off state when the energy stored in transformer T1 isexhausted such as at the transformer reset time. Synchronous rectifiercontroller U2 thus turns on synchronous rectifier switch S2 whencontroller U1 turns off power switch S1 such that the stored energy intransformer T1 is delivered as an output voltage V_OUT across a load asfiltered by output capacitor C1 in conjunction with a pulse of secondarycurrent in secondary winding Ns of transformer T1. For example, adaptivesynchronous rectifier controller U2 may turn on synchronous rectifierswitch S2 responsive to the voltage across synchronous rectifier switchS2 crossing an on-time threshold voltage (e.g., approximately −400 mV).As the energy delivery from transformer T1 is depleted, the secondarycurrent will ramp to zero Amps. The transformer reset point (T1 reset)occurs when the secondary current reaches zero Amps, at which pointsynchronous rectifier controller U2 turns off synchronous rectifierswitch S2. For example, adaptive synchronous rectifier controller U2 mayturn off synchronous rectifier switch S2 responsive to the voltageacross synchronous rectifier switch S2 reaching an off-time thresholdvoltage (e.g., 0 V).

For improved operating efficiencies, synchronous rectifier controller U2may control synchronous rectifier switch S2 to provide functionaloperations similar to a diode device despite the bi-directional currentflow that could occur through synchronous rectifier switch S2. Followingthe transformer reset time and prior to a subsequent cycling on of thepower switch S1, synchronous rectifier controller U2 should maintainsynchronous rectifier switch S2 off. But as discussed previously, theresonant oscillation of the voltage across synchronous rectifier switchS2 following it being cycled off and continuing past the minimumoff-time period may cause this voltage to cross the on-time thresholdvoltage such that the synchronous rectifier switch is then turned on.Since this erroneous cycling on of the synchronous rectifier switch isconducted prior to the cycling on of the power switch S1, the off-timethreshold voltage will be promptly violated after expiration of theminimum on-timer period. Synchronous rectifier controller U2 monitorsthe duration of the on-time period to detect whether it equals theminimum on-time period (or whether it is within a guard band period ofthe expiration of the minimum on-time period) to trigger a faultcondition.

In various embodiments, adaptive synchronous rectifier controller U2 isconfigured to increase the minimum off-time period of synchronousrectifier switch S2 responsive to the detection of the fault conditionto prevent the fault condition from persisting. This advantageousprevention of further episodes of the fault condition may be betterunderstood with consideration of the waveforms shown in FIG. 3.Following an on-time period for power switch S1 in a power cycle 310,controller U1 places power switch S1 in the OFF state, causing thedrain-to-source voltage waveform (V_(D-S)) for the voltage acrosssynchronous rectifier switch S2 to cross an on-time threshold voltage(S2 ON threshold). This threshold crossing causes synchronous rectifiercontroller U2 to place synchronous rectifier switch S2 in the on state.Also at this point, the minimum ON-time timer (S2 MIN T_(on) timer) isinitiated. Once all the energy has been delivered to the secondaryoutput stage 220 and the secondary current reaches zero Amps, theV_(D-S) waveform resonantly rises subsequent to the minimum on-timeperiod so as to cross the off-time threshold voltage (S2 OFF threshold).The V_(D-S) waveform does not trigger the on-time threshold voltageuntil the next power cycle 320 begins, thus representing propersynchronous rectifier control.

During power cycle 320 of power switch S1, controller U1 cycles powerswitch S1 off, which causes the V_(D-S) waveform to cross the on-timethreshold voltage. This threshold crossing in turn causes synchronousrectifier controller U2 to cycle synchronous rectifier switch S2 on. Atthe same time, the minimum ON-time timer is initiated. Once all theenergy has been delivered to the secondary output stage 220 and thesecondary current reaches zero Amps, the V_(D-S) waveform rises so as toexceed the off-time threshold voltage such that the synchronousrectifier switch S2 is cycled off.

Following the minimum off-time period, resonant oscillation of theV_(D-S) waveform may cause the V_(D-S) waveform to cross the on-timethreshold voltage at time 322 so as to cause a premature cycling on ofthe synchronous rectifier switch S2. This also restarts the minimumON-time timer. Since all the energy in transformer T1 has beendelivered, the V_(D-S) waveform rapidly reaches the off-time thresholdvoltage such as during the minimum on-time period. Synchronous rectifierswitch S2 may thus be cycled off when the minimum ON-time timer hasexpired. Synchronous rectifier controller U2 detects a fault conditionin response to synchronous rectifier switch S2 turning OFF at the sametime as the minimum ON-time timer expiring at time 324. This faultcondition is caused by the ringing of the V_(D-S) waveform, which causesthe V_(D-S) waveform to trigger the on-time threshold voltage after theminimum OFF-time timer has expired and prior to the next power cycle. Inorder to prevent the persistent occurrence of this fault condition,controller U2 increases the minimum off-time period such as shown forperiod 330.

A method of operation will now be discussed with reference to aflowchart shown in FIG. 4. Following a start of the method, synchronousrectifier switch S2 is turned on when a V_(D-S) waveform crosses theon-time threshold voltage (V_(TURN) _(_) _(ON)) at a time Ta an act 402.In that regard, synchronous rectifier controller U2 monitors V_(D-S)(e.g., the V_(D-S) waveform), such as by using voltage sensor 122 (shownin FIG. 1). In response to V_(D-S) triggering the on-time thresholdvoltage, synchronous rectifier controller U2 also initiates the minimumON-time timer in act 402. The synchronous rectifier switch must thus bemaintained on for at least the minimum on-time period, which expires ata time Tb in an act 404.

An act 406 occurs subsequent to the expiration of the time Tb at a timeTc. In act 406, the synchronous rectifier controller reacts to the theV_(D-S) waveform crossing the time-off threshold voltage (V_(TURN) _(_)_(OFF)) by turning off the synchronous rectifier switch. Time Tc thussignifies the termination of the on-time period following time Ta.Control circuit 226 (FIG. 2) may thus test the duration of this on-timeperiod in an act 408 by determining whether the difference between Tcand Tb is less than or equal to a threshold time duration Td. In otherwords, act 408 is asking whether the on-time period for the synchronousrectifier switch following time Ta was anomalously short. Such ananomalously short on-time period is shown between times 322 and 324 inFIG. 3 due to the resonant oscillation of the drain-to-source voltagefor the synchronous rectifier switch following the switch off-time.

If the determination in act 308 is positive, control circuit 226 (FIG.2) increases the duration of the minimum off-time period and enables theoff-time timer with this revised minimum off-time period in an act 410.Conversely, an on-time period following the cycling off of the powerswitch has a normal (not anomalously short) duration as shown in FIG. 3for both power cycles 310 and 320. In such a case, the determination inact 408 would be negative such that the off-time timer is enabled in anact 412 using an unchanged value of the minimum off-time period.

In some embodiments, synchronous rectifier controller U2 detects thefault condition responsive to V_(D-S) reaching, exceeding, and/ortriggering the off-time threshold voltage before or at the same time asthe minimum ON-time timer expiring. For example, synchronous rectifiercontroller U2 may detect the fault condition responsive to V_(D-S)reaching the S2 OFF voltage threshold before or at the same time as theminimum ON-time timer expiring. In alternative embodiments, synchronousrectifier controller U2 may detect the fault condition responsive toV_(D-S) exceeding and triggering the off-time threshold voltage beforethe expiration of a period equalling a sum of the minimum on-time periodand a guard-band period (T_(C)). This may be represented as follows:

IF: T _(ON) ≦T _(ON) _(_) _(MIN) +T _(C)

THEN: Increase T_(OFF) _(_) _(MIN).

The duration of the guard-band period is a design choice. In someembodiments, the guard-band period may have a duration that is less thanor equal to 10% of the minimum on-time period.

As those of some skill in this art will by now appreciate and dependingon the particular application at hand, many modifications, substitutionsand variations can be made in and to the materials, apparatus,configurations and methods of use of the devices of the presentdisclosure without departing from the spirit and scope thereof. In lightof this, the scope of the present disclosure should not be limited tothat of the particular embodiments illustrated and described herein, asthey are merely by way of some examples thereof, but rather, should befully commensurate with that of the claims appended hereafter and theirfunctional equivalents.

We claim:
 1. A switching power converter, comprising: a synchronousrectifier switch coupled to a secondary winding of a transformer; and asynchronous rectifier controller coupled to the synchronous rectifierswitch and configured to: monitor a voltage across the synchronousrectifier switch with regard to a an on-time threshold voltage forcycling the synchronous rectifier switch on and with regard to anoff-time threshold voltage for cycling the synchronous rectifier switchoff, wherein the controller is configured to maintain the synchronousrectifier switch on for an on-time period equaling at least a minimumon-time period following each cycling on of the synchronous rectifierswitch and to maintain the synchronous rectifier switch off for at leasta minimum off-time period following each cycling off of the synchronousrectifier switch; detect a fault condition with regard to a duration ofthe on-time period for the synchronous rectifier switch; and increasethe minimum off-time period for the synchronous rectifier switch inresponse to the detection of the fault condition.
 2. The switching powerconverter of claim 1, wherein the synchronous rectifier controller isconfigured to: monitor a duration of the minimum on-time period using aminimum on-time timer.
 3. The switching power converter of claim 2,wherein the controller is further configured to detect the faultcondition by detecting that a duration of the on-time period equals aduration of the minimum on-time period.
 4. The switching power converterof claim 2, wherein the controller is further configured to detect thefault condition by detecting that a duration of the on-time periodequals a sum of a duration of the minimum on-time period and a guardband period.
 5. The switching power converter of claim 4, wherein theguard band period is no more than 10% of the minimum on-time period. 6.The switching power converter of claim 2, wherein the synchronousrectifier controller is configured to detect the fault conditionresponsive to the synchronous rectifier switch being turned off at whenthe minimum ON-time timer expires.
 7. The switching power converter ofclaim 1, further comprising: a power switch coupled to a primary windingof the transformer; and a primary controller coupled to the power switchand configured to turn on and to turn off the power switch to regulatean output voltage across a load coupled to the secondary winding.
 8. Theswitching power converter of claim 1, wherein the synchronous rectifierswitch is a field-effect transistor (FET).
 9. The switching powerconverter of claim 1, wherein the synchronous rectifier switch is abipolar junction transistor.
 10. The switching power converter of claim1, wherein the controller is configured to increase the minimum off-timeperiod to no more than a maximum amount.
 11. A method comprising:repeatedly switching on and off a synchronous rectifier switch coupledto a secondary winding of a transformer responsive to a voltage acrossthe synchronous rectifier switch, wherein following each switching on ofthe synchronous rectifier switch, the synchronous rectifier switch ismaintained on for an on-time period lasting at least a minimum on-timeperiod, and wherein following each switching off of the synchronousrectifier switch, the synchronous rectifier switch is maintained off forat least a minimum off-time period; detecting a fault condition withregard to a duration of the on-time period; and increasing the minimumoff-time period responsive to the detection of the fault condition. 12.The method of claim 11, wherein each switching on of the synchronousrectifier switch is responsive to the voltage crossing an on-timethreshold voltage.
 13. The method of claim 12, wherein detecting thefault condition comprises detecting that the duration equals the minimumon-time period.
 14. The method of claim 12, wherein detecting the faultcondition comprises detecting that the duration equals a sum of theminimum on-time period and a guard band period.
 15. The method of claim14, wherein the guard band period is less than or equal to 10% of theminimum on-time period.
 16. The method of claim 12, further comprisingcycling a power switch coupled to a primary winding of the transformerto regulate a voltage across a load coupled to the secondary winding.17. A synchronous rectifier controller, comprising: a voltage sensorconfigured to sense a voltage across a synchronous rectifier switch; acontroller configured to cycle the synchronous rectifier switch on for aon-time duration equaling at least a minimum-on time period responsiveto the sensed voltage crossing an on-time threshold voltage; and a firsttimer configured to time the minimum on-time period, wherein thecontroller is further configured to detect a fault condition responsiveto the on-time duration being less than or equal to a guard band periodfollowing an expiration of minimum on-time period as timed by the timer,and wherein the controller is further configured to increase a minimumoff-time period for the synchronous rectifier switch responsive to thedetection of the fault condition.
 18. The synchronous rectifiercontroller of claim 17, wherein the guard band period equals is lessthan or equal to 10% of the minimum on-time period.
 19. The synchronousrectifier controller of claim 17, wherein the controller is furtherconfigured to increase the minimum off-time period to no more than amaximum amount.
 20. The synchronous rectifier controller of claim 17,further comprising a second timer configured to time the minimumoff-time period.